1. Field of the Invention
The present invention relates to a method for forming a conductive strap. In particular, the present invention relates to a method for forming a conductive strap on the surface of a substrate.
2. Description of the Prior Art
A DRAM unit is made of a metal oxide semiconductor (MOS) transistor in series connection with a capacitor. In the current DRAM process, the capacitor is designed to be a stack capacitor on the surface of the substrate or a deep trench capacitor buried in the substrate.
Because the deep trench capacitor is deeply buried in the substrate, conventionally a buried strap is used to electrically connect the drain and the deep trench capacitor in the MOS. However, in order to avoid the strong electric field caused by the drain and the buried strap as the electrical connection in the MOS to damage the performance of the devices, such as the raise of the dark current, the gate channel region is accordingly arranged in a deeper trench to keep a distance from the strong electric field in the vicinity of the buried strap as far away as possible.
However, in addition to the fact that it is well known that it is not an optimal solution since it is very complicated and difficult to set the gate channel region in a gate trench of high aspect ratio plus the difficulty to control such device, high integration, shrinkage of the dimension of the capacitor as well as the shorter and shorter distance of the neighboring elements all cause the electric resistance of the buried strap to dramatically increase. Besides, the method for forming the conventional single buried strap at the collar of the trench capacitor in the trench capacitor DRAM structure is complicated and needs improving.
Therefore, it is needed to provide a novel method for forming the conductive strap. On one hand the gate channel region may keep a distance from the strong electric field in the vicinity of the buried strap as far away as possible and on the other hand the conductive strap may have a larger contact area as well as to omit the conventional single buried strap at the collar of the trench capacitor to simplify the process.